Ref ID : 5731
David J. Hibberd; Ultrastructure of the colonial colourless zooflagellates Phalansterium digitatum Stein (Phalansteriida Ord. Nov.) and Spongomonas uvella Stein (Spongomonadida Ord. Nov.). Protistologica XIX(4):523-535, 1983
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The structure of the colourless colonial flagellate Phalansterium digitatum Stein has been examined by electron microscopy. The cylindrical cells have one smooth flagellum, the proximal part of which is closely surrounded by a simple protoplasmic collar; there is no trace of a second flagellum or a supernumerary basal body. The dichotomously branched tubes comprising the colony consist of a mucilaginous matrix in which are embedded small spherules which are produced in cytoplasmic vesicles. The nucleus occupies a central position in the cell, distant from the parabasal Golgi body, the forming face of the latter lying distally, close to the descending flagellar root. The mitochondria have tubular cristae, and a kinetoplast is absent. A concentric series of dense bands surrounds the flagellar basal body and gives rise to a radiating array of about 60 microtubules. The transition region and other features of the flagellar apparatus are described in detail. Supplementary observation on Spongomonas uvella Stein confirm and extend previous EM observations. The phylogenetic affinities of Phalansterium are examined and it is concluded that this genus is related neither to the choanoflagellates, with which it has generally been classified, nor to Rhipidodendron and Spongomonas, which it superficially resembles in habit and colony structure. Phalansterium and Rhipidodendron/Spongomonas cannot be considered either to be related to any of the algal classes or to belong to any of the orders comprising the Zoomastigophorea and, accordingly, the new protozoan orders Phalansteriida and Spongomonadida are erected to accommodate them.